Will droughts hurt less if plants aren’t so thirsty?

Researchers say popular long-term drought estimates have a major flaw: They ignore that plants will be less thirsty as carbon dioxide rises. A new study shows that shifts in how plants use water could roughly halve the extent of climate change–induced droughts.

“Plants matter,” says Abigail Swann, an assistant professor of atmospheric sciences and biology at the University of Washington. “A number of studies assume that plant water needs are staying constant, when what we know about plants growing in lots of carbon dioxide suggests the opposite.”

Recent studies have estimated that more than 70 percent of our planet will experience more drought as carbon dioxide levels quadruple from pre-industrial levels over about the next 100 years. But when Swann and her coauthors account for changes in plants’ water needs, this falls to 37 percent, with bigger differences concentrated in certain regions.

The reason is that when Earth’s atmosphere holds more carbon dioxide, plants actually benefit from having more of the molecules they need to build their carbon-rich bodies. Plants take in carbon dioxide through tiny openings, called stomata, that cover their leaves. But as they draw in carbon dioxide, moisture escapes. When carbon dioxide is more plentiful, the stomata don’t need to be open for as long, and so the plants lose less water. The plants thus draw less water from the soil through their roots.

Global climate models already account for these changes in plant growth. But many estimates of future drought use today’s standard indices, like the Palmer Drought Severity Index, which only consider atmospheric variables such as future temperature, humidity, and precipitation.

“I had a very strong suspicion that you would get a different answer if you considered how the plants were responding,” Swann says.

The study compares today’s drought indices with ones that take into account changes in plant water use. It confirms that reduced precipitation will increase droughts across southern North America, southern Europe, and northeastern South America. But the results show that in Central Africa and temperate Asia—including China, the Middle East, East Asia, and most of Russia—water conservation by plants will largely counteract the parching due to climate change.

“In some sense there’s an easy solution to this problem, which is we just have to create new metrics that take into account what the plants are doing,” Swann says. “We already have the information to do that; we just have to be more careful about ensuring that we’re considering the role of the plants.”

Is this good news for climate change? Although the drying may be less extreme than in some current estimates, droughts will certainly increase, researchers says, and other aspects of climate change could have severe effects on vegetation.

“There’s a lot we don’t know, especially about hot droughts,” Swann says. The same drought at a higher temperature might have more severe impacts, she notes, or might make plants more stressed and susceptible to pests.

“Even if droughts are not extremely more prevalent or frequent, they may be more deadly when they do happen,” she says.

The coauthors are from Oak Ridge National Laboratory, Lawrence Berkeley National Laboratory, and the University of California, Irvine. Funding came from the National Science Foundation and the US Department of Energy.